1. Field of the Invention
The present invention relates to a piezoelectric element drive circuit configured to apply to a piezoelectric element two drive signals with opposite phases.
2. Description of the Related Art
Various types of drive circuits for generating drive signals applied to piezoelectric elements have been designed. For instance, Japanese Unexamined Patent Application Publication No. 2008-89308 discloses a drive circuit for generating drive signals, which drive a piezoelectric element at a resonance frequency thereof, by employing a positive feedback loop.
Apart from Japanese Unexamined Patent Application Publication No. 2008-89308, an H bridge circuit has also been practiced as a circuit for supplying two drive signals, having reversed phases, to a load, for example, a motor. The H bridge circuit has the merit that because the drive signals having a voltage equal to twice a source voltage is obtained, the drive voltage can be increased while power consumption is low. Accordingly, when a piezoelectric element is driven with a low-voltage power source, e.g., a cell, it is effective to assemble the H bridge circuit in the self-excited drive circuit disclosed in Japanese Unexamined Patent Application Publication No. 2008-89308, and to supply drive signals to the piezoelectric element from two output terminals of the H bridge circuit.
However, because the H bridge circuit is a switch control circuit, the drive signals output from the H bridge circuit are each a rectangular wave. Furthermore, because the drive signals output from the H bridge circuit are each a rectangular wave and the drive circuit is a self-excited circuit, a control signal input to the H bridge circuit again through feedback is also a rectangular wave. Stated in another way, in this type of drive circuit, a signal transmitted through a feedback loop is a rectangular wave, and a drive waveform applied to the piezoelectric element is constantly a rectangular wave as a matter of course.
Here, the rectangular wave has a waveform with a sharp voltage change rate (hereinafter referred to as a “slew rate”) per unit time, and it has not only a frequency component at the resonance frequency of the piezoelectric element, but also frequency components over a very wide range. In other words, the rectangular wave includes not only the frequency component that contributes to substantial operation of the piezoelectric element, but also the frequency components, such as harmonics of the resonance frequency of the piezoelectric element, which do not contribute to the substantial operation and which just consume electric power by merely causing resonance.
It is, therefore, not easy to effectively reduce the power consumption just by combining the self-excited drive circuit and the H bridge circuit with each other.